The present invention generally relates to monitoring stability margins and responding appropriately to instability in closed loop systems and, more particularly, to monitoring stability margins in servo control systems of digital data storage devices.
Disk drives are digital data storage devices that can store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes a plurality of magnetic recording disks that are mounted to a hub of a spindle motor and rotated at a high speed. A plurality of actuator arms including read/write transducers disposed at ends thereof are disposed adjacent to surfaces of the disks. The transducers are used to transfer data between the disks and a host computer. The transducers can be radially positioned over the disks by a rotary actuator and a closed loop, digital servo system, and can fly proximate the surfaces of the disks upon air bearings.
A plurality of nominally concentric tracks can be defined on each disk surface. A preamp and driver circuit generates write currents that are used by the transducer to selectively magnetize the tracks during a data write operation and amplifies read signals detected by the transducer from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are connected to the preamp and driver circuit to transfer the data between the disks and the host computer.
The servo system can operate in two primary modes: seeking and track following. During a seek operation, a selected transducer is moved from an initial track to a destination track on the corresponding disk surface. The servo system applies current to an actuator coil to first accelerate and then decelerate the transducer toward the destination track.
The actuator arms can be moved very quickly in response to the actuator current. The acceleration of the actuator in response to the applied current excites resonances, or flexible modes, of the actuator. Vibrations at these modes may be difficult to attenuate and/or may interfere with drive operation. For example, if the actuator arms are excited by signals having frequency components at the suspension modes, the system could experience large tracking errors, problems with off-track detection algorithms, and/or instability. Accordingly, many servo control systems use one or more notch filters in the control loop to attenuate the effects of these resonances. Plant is a generic term used in control systems to refer to the mechanical system being controlled. It will be appreciated that in the context of a disk drive, the “plant” refers to the electromechanical device that positions the transducers adjacent the tracks in response to a control signal, and includes the actuator coil, actuator arms and transducers.
The term “system mode” is used herein to identify the first major flexible mode in a plant transfer function for an actuator of a hard drive. The shape of the system mode of a disk drive (i.e. the pattern in which the mechanical components of the plant vibrate) may be a consequence of both translation of the actuator bearing as well as in-phase sway of the actuator arms and coil. The system mode can be identified from a closed loop plant transfer function. Typically, the mechanical components of the actuator are designed to have the system mode at a particular frequency, and the notch filter(s) are designed accordingly.
In a mass production environment, the actual types of actuators used in hard disk drives may change for various reasons. The introduction of new vendors, process variation at the manufacturing site, and/or design modifications may alter the family of plants. Different actuators will have different suspension modes due to differences in design and/or manufacture thereof. High frequency suspension modes of the actuator may be of special interest, because if these modes change, the notch filters may not adequately attenuate the effects of these resonances that may cause the problems described above. Changes in the suspension modes of a disk drive may also provide an early warning of a problem at the component level.